Also, I guess I'm different from @antimonysarah in the sense that I prefer the flatter roads over rollers when I am tired.

I was surprised at my reaction Our local rides are all at least rolling hills the whole way, so I'd never been coming in on long, straight, flat roads in the night. It was definitely more mental than physical exhaustion; every time I'd finally get to a turn and realize I had 10 straight miles on the next road too I felt like I was getting further, not closer, to the end.